The invention relates generally to well drilling equipment and, more specifically, to a hoist or drawworks for well drilling.
Well drilling involves the use of many large and heavy items, for example, drill collars, pipe, well casing, etc. To use these items effectively, the items must be lifted and moved. Because of the size and weight of these items a large tower, referred to as a derrick or mast, is erected. A block and tackle arrangement is installed at the top of the tower. Wire rope or cable is reeved or strung through the sheaves or pulleys of the block and tackle arrangement.
The block and tackle arrangement provides a mechanical advantage, allowing a relatively small force to be used to lift relatively heavy objects. However, this mechanical advantage involves a trade-off; the wire rope or cable is pulled a much longer distance than the distance that the load supported by the block and tackle arrangement moves. Also, the block and tackle arrangement introduces additional friction into the system, thereby reducing its efficiency.
Because of the long distance that the wire rope or cable must travel and the great weight involved, a hoist or drawworks is used. The hoist or drawworks has a drum for reeling the wire rope or cable in or out. The drum is mounted on a drum shaft. The drum shaft is coupled to a motor or prime mover through a transmission. The motor and transmission provide the force to rotate the drum and reel in the wire rope or cable.
The force provided by the motor and transmission needs to be sufficient to overcome the weight of the items being lifted, as well as any friction or other inefficiencies in the system. Since the motor and transmission have finite limits on the amount of force they can provide, and the wire rope or cable also has limits on the amount of force they can withstand, it is important to obtain an indication of the actual force present at the load.
Since the load may include a drill string extending a great distance into the well hole, numerous factors may contribute to the amount of force present at the load. When the load is static, the weight to the drill string and the traveling block of the block and tackle arrangement contribute to the force at the load. However, if, for example, the well hole is drilled so as to deviate from vertical, some of the weight of the drill string may be borne by the lower side of the angled region of the well hole. When the load is being raised or lowered, dynamic factors affect the force on the load. For example, friction between the drill string and the drill hole may increase the force needed to raise the load. Friction in the block and tackle arrangement may also increase the force needed to raise the load by effectively preventing some of the force applied by the hoist or drawworks from reaching the actual load.
To prevent damage to the equipment and to accurately control the forces being applied, techniques for measuring force are used. The end of the wire rope or cable opposite the hoist or drawworks as it comes from the block and tackle arrangement is referred to as a dead line. The dead line is anchored by a dead line anchor to a fixed location. The dead line anchor is provided with a force transducer to measure the force or tension on the dead line. However, because of friction in the block and tackle arrangement and energy needed to bend the wire rope or cable as it passes through the block and tackle arrangement, the amount of force or tension measured at the dead line does not, under dynamic conditions, accurately reflect the amount of force on the wire rope or cable leading from the block and tackle arrangement to the hoist or drawworks, which is referred to as the fast line.
The force or tension on the fast line is usually greater than the force or tension on the dead line when the load is being raised and less than the force or tension on the dead line when the load is being lowered. These differences are often approximately plus or minus 15 percent of the actual force on the load. The differences increase exponentially with the number of lines through the block and tackle arrangement or the number of sheaves or pulleys in the block and tackle arrangement.
The force on the load could be determined if the force or tension on both the fast line and the dead line were known. Unfortunately, while the force or tension on the dead line can be easily measured at the dead line anchor, the force or tension on the fast line is difficult to measure because of its motion.
Alternative approaches have been developed to measure the force on the load. Since friction in the block and tackle arrangement can be assumed to be fairly evenly distributed, the force on the crown block or middle line of the block and tackle arrangement can be measured. Since the middle line has the same number of sheaves or pulleys between it and the fast line as it has between it and the dead line, the frictional losses are approximately equally distributed on both sides and effectively cancel out each other. Unfortunately, this technique requires that the force transducer be located in the block and tackle arrangement, which is mounted at the top of the tower. Since the tower may be, for example, 200 feet high, the force transducer is relatively inaccessible, making it difficult to install and maintain. Also, the signals from the force transducer must be delivered down the tower to operators or equipment below. Communication of the signals is difficult to achieve accurately and reliably.
Another alternative approach is to install a pad-type strain gauge at one of the legs of the tower. The pad-type strain gauge senses indicative of force on the tower exerted by force on the load. This technique is difficult to implement because it requires integrating a strain gauge into the base of the tower, which is an immense and massive structure. As a result, installation and maintenance of the strain gauge is difficult.
Thus, a technique is needed to accurately determine the force on a load without the difficulties and disadvantages of the prior art techniques.
The invention provides a method and apparatus for measuring the torque applied to the drum shaft of a hoist. By measuring the torque on the drum shaft, the force or tension on the fast line can be accurately determined. If the force or tension on the dead line is also measured, the forces on the fast line and dead line can be used to determine the force applied to the load.
One embodiment of the invention uses a transmission coupled to the drum shaft as a moment arm. The transmission is coupled to a fixed point by a strain-sensing element located some distance from the center of the drum shaft. The distance between the center of the drum shaft and the point along the transmission where the strain-sensing element is mounted provides the moment arm for measuring the torque on the drum shaft.
While the invention may be practiced with strain-sensing elements, such as electrical strain gauges, that can operate effectively without any substantial motion, other types of strain-sensing elements, such as hydraulic load cells, can also be used. Any movement of the transmission allowed by the strain-sensing element can be accommodated by a flexible gear tooth coupling between the motor or prime mover and the transmission. One example of such a flexible gear tooth coupling uses gears having spherically curved teeth to accommodate motion between the motor and transmission. Other techniques for accommodating motion between the motor and transmission may also be used. For example, elastomeric motor mounts could be used to mount the motor on its mounting surface.
Another embodiment of the invention provides xe2x80x9cCxe2x80x9d-shaped side plates to support and mount the main bearings of the drum shaft. The cutout provided by the xe2x80x9cCxe2x80x9d-shape of the side plates allows the drum shaft, drum shaft bearings, and drum shaft bearing carriers to be passed from outside the side plates to inside the side plates without the need to remove components from the ends of the drum shaft. Once the drum shaft and its bearing components are located within the cutout portions of the xe2x80x9cCxe2x80x9d-shaped side plates, the bearing carriers are bolted to the side plates so as to locate the drum shaft at the proper location relative to the side plates.
With the drum shaft in place, a plate or link installed to span the cutout of each side plate. The plate or link is coupled to the side plate on each side of the cutout region. For example, a link having an elongated xe2x80x9cHxe2x80x9d-shape may be used to span the gap of the cutout region. The ends of the link form a clevis-type arrangement, allowing a pin to be inserted through one side of the link, through the side plate, and through the other side of the link. A pin is inserted through each end of the link to couple each end of the link to the side plate on its respective side of the cutout region.
Using a pin, bolt, or other fastener of round cross section to connect the link to the side plate allows the link to pivot away from the cutout in the side plate when one of the fasteners is removed. Thus, the link serves as an easily releasable link to strengthen and stabilize the side plates while allowing easy access to the drum shaft and its bearing components for installation, removal, or maintenance.